Spinning, doping, dedoping and redoping polyaniline fiber

ABSTRACT

A composition of matter suitable for spinning polyaniline fiber, a method for spinning electrically conductive polyaniline fiber, a method for exchanging dopants in polyaniline fibers, and methods for dedoping and redoping polyaniline fibers are described.

RELATED CASES

This application is a Continuation Application of patent applicationSer. No. 12/629,759, which was filed on Dec. 2, 2009, and issued as U.S.Pat. No. 7,897,082 on Mar. 1, 2011, and which was a Divisional PatentApplication of Non Provisional patent application Ser. No. 10/672,323,which was filed on Sep. 26, 2003, and patented on Dec. 8, 2009 as U.S.Pat. No. 7,628,944, and which claims the benefit of Provisional PatentApplication Ser. No. 60/423,092, for “Polyaniline Fiber” filed on Oct.30, 2002, and of Provisional Patent Application Ser. No. 60/495,493 for“Doping, Dedoping and Redoping Polyaniline Fiber” filed on Aug. 15,2003, the disclosure and teachings of which applications are herebyincorporated by reference herein.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.MDA972-99-C-0004 and under Contract No. NBCHCO20069 awarded by the U.S.Defense Advance Research Projects Agency to Santa Fe Science andTechnology, Inc., Santa Fe, N. Mex. 87507. The government has certainrights in the invention.

FIELD OF THE INVENTION

The present invention is related generally to polymeric fibers and, moreparticularly, to the spinning of polyaniline fibers and the manipulationof dopants therein to achieve desired fiber characteristics.

BACKGROUND OF THE INVENTION

Successful processing of polyaniline emeraldine base (PANI-EB) intouseful high strength and high conductivity fibers requires solutionsthat are suitable for continuous fiber production. In “ConductivePolymer Compositions” by Phillip Norman Adams et al., InternationalPublication Number: WO 99/24991, published on 20 May 1999, a fluidconductive mixture for use in the preparation of coatings, films andfibers based on polyaniline in base form doped with a sulfonic acidhaving in addition to at least one sulfonic acid group a secondhydrogen-bonding functional group dispersed in an acid solvent having apK_(a) less than 4.5 but substantially higher than that of the sulfonicacid, is described. Specific examples of2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) as the sulfonicacid and dichloroacetic acid (DCAA) spun into a competitive solvent inwhich the DCAA is soluble, but in which polyaniline is not soluble aredisclosed. The ratio of the number of AMPSA molecules to the number ofnitrogen atoms in the polyaniline as a reference was between 0.3 and1.0; typically this ratio was 0.6. Dry polyaniline powder (M_(w)˜150,000g·mol⁻¹) was ground with AMPSA and added over a 5 min. period to theDCAA under flowing nitrogen in a glove box to form a mixture having 9mass % solids. The solution pressurized with nitrogen was extruded intothe coagulant at 50±5° C. Adams et al. stated that the maximum solidscontent at which level gelation is not experienced is 5 mass %.

In “Inherently Electrically Conductive Fibers Wet Spun from a SulfonicAcid-Doped Polyaniline Solution” by Stephen J. Pomfret et al., Adv. Mat.10, 1351 (1998), it is stated that: “The primary alternative method ofproducing conductive polyaniline involves processing from theelectrically insulating emeraldine base (EB) form, then post-doping withan aqueous protonic acid. There are disadvantages of this method: inmost cases the resulting material is doped inhomogeneously; it isdedoped relatively easily; and the materials properties are usuallyadversely affected on doping. Processing from an inherently conductivesolution, however, results in homogeneous doping, and the bulky sulfonicacids cannot be easily removed from the material afterwards.”

It is known that the performance of conducting-polymer-based devices isdependent on the properties of the dopant anion. As an example, whenpolyaniline fibers are used in electrochemical devices with non-aqueouselectrolytes (e.g. organic solvent or ionic liquid), the sulfonic acidthat is used to solublize the doped form of the polymer to enable fiberproduction (for example, AMPSA) inhibits the performance of the device.As stated in “Long-Lived Conjugated Polymer Electrochemical DevicesIncorporating Ionic Liquids,” International Publication Number WO02/063073 A1, as-spun AMPSA-doped polyaniline (PANI.AMPSA) fiber is apoor choice for the active electrode in electrochemical devicescontaining an organic solvent electrolyte (for example, LiPF₆ dissolvedin propylene carbonate); that is, weak electroactivity of such fibersmay be deduced from the low observed currents in cyclic voltammogramsthereof. Moreover, little actuation (change of length) has been observedfor such fibers when a voltage is applied thereto. This is related tothe large size of the AMPSA anion and, consequently, a low diffusioncoefficient, thereby rendering the anions unable to exchange with thePF₆ ⁻ anions in the electrolyte.

Furthermore, for certain membrane-based separation applications,specific dopant anions lead to enhanced performance of polyanilinemembranes. See, for example, U.S. Pat. No. 5,358,556 for “MembranesHaving Selective Permeability” which issued to Kaner et al. on Oct. 25,1994.

Accordingly, it is an object of the present invention to prepare stable,high solids content spinning solutions from high molecular weightpolyaniline.

Another object of the invention is to provide a method for spinningpolyaniline fiber.

Yet another object of the present invention is to provide a method forpartially or totally replacing dopants present in as-spun polyanilinefibers with selected dopants in order to achieve desired characteristicsof the fibers.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention as embodied and broadly describedherein, the composition of matter hereof includes between 6 and 14 mass% of an AMPSA/polyaniline mixture, and between 0.1 and 0.6 mass % ofwater in DCAA, where there are between 30 and 100 AMPSA molecules per100 aniline repeat units of the polyaniline.

In another aspect of the present invention, in accordance with itsobjects and purposes, the method for spinning polyaniline fiber hereofincludes: adding between 6 and 14 mass % of a mixture of AMPSA andpolyaniline containing between 2 and 12 mass % of water to DCAA, suchthat there are between 30 and 100 molecules of AMPSA per 100 anilinerepeat units of the polyaniline, forming thereby a composition, wherebythe temperature of the composition does not rise above about 35° C.;extruding the composition through a spinneret into a nonsolvent orcoagulant for the polyaniline, thereby forming a polyaniline fiber.

In yet another aspect of the present invention, in accordance with itsobjects and purposes, the method for exchanging dopant molecules inelectrically conductive fibers spun from a solution includingpolyaniline, AMPSA and DCAA with a selected dopant molecule hereof,includes extruding the spin solution into a coagulant, thereby causingthe spin solution to coagulate and form a fiber, and immersing theresulting fiber in a solution containing the selected dopant moleculefor a time effective to achieve dopant exchange.

In still another aspect of the present invention, in accordance with itsobjects and purposes, the method for removing dopant molecules fromelectrically conductive fibers spun from a solution comprisingpolyaniline, AMPSA and DCAA hereof, includes extruding the spin solutioninto a solution containing a coagulant, thereby causing the spinsolution to coagulate and form a polyaniline fiber, and immersing theresulting polyaniline fiber in a solution effective for removing thedopant molecules for a time sufficient to achieve a chosen level ofdopant molecule removal.

In another aspect of the present invention, in accordance with itsobjects and purposes, the method for redoping electrically conductivefibers spun from a solution comprising polyaniline, AMPSA and DCAA witha selected dopant molecule hereof, includes extruding the spin solutionin a solution containing a coagulant, thereby causing the spin solutionto coagulate and form a polyaniline fiber, immersing the resulting fiberin a solution effective for removing the dopant molecules for a timesufficient to achieve a chosen level of dopant molecule removal, andimmersing the dedoped polyaniline fiber in a solution containing theselected dopant molecules for a time effective for achieving a chosenlevel of selected dopant molecules in the polyaniline fiber.

Benefits and advantages of the invention include the ability tocontinuously spin polyaniline fiber having selected properties, and tochange these properties by manipulating the dopant molecules in thepolyaniline fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiments of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 a is a scanning electron microscope (SEM) micrograph of a 28 μmsolid polyaniline fiber spun in accordance with the teachings of thepresent invention, while FIG. 1 b is a SEM micrograph of a yarn formedfrom 20 polyaniline fibers having a twist ratio of 7 turns per inch.

FIG. 2 is a graph of fiber diameter as a function of fiber stretch ratiofor fiber spun from the A solution.

FIG. 3 is a graph of the percent extension of the fiber at break as afunction of stretch ratio for fiber spun from the A solution.

FIG. 4 is a graph of fiber peak stress as a function of stretch ratiofor fiber spun from the A solution.

FIG. 5 is a graph of fiber modulus as a function of stretch ratio forfiber spun from the A solution.

FIG. 6 is a graph of fiber conductivity as a function of stretch ratiofor fiber spun from the A solution.

FIG. 7 is a graph of typical fiber stress/strain curves for as-spunfibers as a function of stretch ratio for fiber spun from the Asolution.

FIG. 8 is a graph of the conductivities as a function of temperature foras-spun AMPSA-doped polyaniline fiber, fiber dedoped using steam, andfiber redoped with methanesulfonic acid.

FIG. 9 a is a cyclic voltammogram for a 10 mm length of polyanilinefiber redoped with triflic acid, while FIG. 9 b is a cyclic voltammogramof polyaniline fibers obtained in the ionic liquid electrolyte,1-butyl-3-methylimizadolium hexafluorophosphate at a scan rate of 5mV·s⁻¹ for a 10 mm length of the as-spun fiber doped with AMPSA.

FIG. 10 is a graph of doping level as a function of conductivity of thepolyaniline fibers redoped with triflic acid.

FIG. 11 is a graph showing percent weight loss as a function oftemperature for as-spun, AMPSA-doped polyaniline fiber (PANI.AMPSAfiber), for the as-spun, AMPSA-doped polyaniline fiber (PANI.AMPSA),after dedoping the as-spun fiber with ammonium hydroxide (EB fiber), andfor the dedoped fiber redoped with phosphoric acid (PANI.H₃PO₄ fiber)and redoped with triflic acid (PANI.triflate fiber).

DETAILED DESCRIPTION

Briefly, the present invention includes a method for preparing stablesolutions of emeraldine base polyaniline (PANI-EB) and2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) in dichloroaceticacid (DCAA) [PANI-EB-AMPSA-DCAA] dope solutions having high solidscontent, and the characterization of these solutions with respect totheir properties over time, as well as characterization of the short andlong term conductivities and mechanical properties of the resultingas-spun fibers.

Wet fiber spinning of polyaniline in accordance with the presentinvention includes: (1) preparation of stable spinning solutions, alsotermed the “spinning dope” or “dope”; (2) extruding the dope underpressure through a spinneret into a coagulating solvent, wherein thedope solvent is exchanged with the coagulant, causing precipitation andsolidification of the polyaniline into a fiber; (3) collecting theformed polyaniline fiber onto a series of take-up drums (called godets),where further extraction of the dope solvent occurs and/or chemicalmodification of the fiber takes place; (4) stretching the fiber betweenat least two godets such that the mechanical and electrical conductivityproperties of the fiber are altered by inducing structural anisotropy;and (5) winding the fiber onto packages or bobbins. While the abovedescribed process is referred to as wet-spinning, it will be apparent toone skilled in the art that the polyaniline solutions described hereincan be readily adapted to other fiber manufacture processes including,as examples, dry-spinning, dry-jet wet spinning or air-gap spinning, andgel spinning.

In the wet spinning process, polyaniline fibers are formed by extrudingnarrow diameter streams of the PANI-AMPSA-DCCA spinning solution througha spinneret (extrudate) into a coagulant. The solution is first filteredto remove small solid particles that would block the orifice of thespinneret, before being forced under pressure through the holes of thespinneret using a gear pump or other suitable device. A spinneret is adie having one or more holes through which the solution is extruded intothe coagulation bath. Spinnerets used for industrial fiber spinningtypically have between 50 and 200,000 holes with diameters ranging from10 to 1000 μm. The shape of the orifice of the spinneret dictates thegeometrical shape of the cross-sectional area of the fiber in thesolid-state. Clearly, many spinneret shapes can be used to process thefibers of the present invention; however, the processing of solid fibersand hollow fibers are described hereinbelow.

The downstream side of the spinneret is situated either above or belowthe liquid surface of the coagulation bath, in order that the spinningdope exiting the spinneret enters the coagulant immediately or soonafter exiting. Formation of the polyaniline fiber occurs rapidly as themixture enters the coagulation bath and contacts the nonsolvent whichextracts or withdraws the spinning solvent from the jets of spinningsolution exiting each spinneret hole causing the polymer tosupersaturate the spinning solution and to precipitate as solids in theshape of fibers.

As the solvent for the dope solution (DCAA) diffuses from the formingfibers in the coagulation bath, the polymer continues to precipitate andto form a semi-solid fiber. While still in the coagulation bath, thefiber achieves sufficient cohesion and strength to remain unbroken uponremoval from the bath. As the solvent diffuses out of the extrudate intothe bath, the polymer precipitates initially as a gel at theextrudate-coagulant interface but progressively throughout the fiberspinning process, the extrudate approaches its final fiber properties.Coagulation rate, concentration and temperature of the spinningsolution, composition, concentration and temperature of the coagulatingliquid; and the stretch applied during spinning between godets areprocessing variables which influence the gel structure and the finalfiber properties.

The described spinning process is based on precipitation in thecoagulation bath, without chemical modification of the newly formedfiber; however, modifications may be instituted subsequently, as theformed fiber is contacted with solutions in the godet baths. The fibersare either continuously or periodically removed from the coagulationbath by a take-up or pick-up roll, or godet, followed by a series offurther processing operations, including immersion in a series ofcoagulation baths, washing, wet stretching or orientational drawing,drying, and optionally, hot stretching and annealing to produce selectedphysical properties for use in textile materials. This furtherprocessing of the fiber leads to greater uniformity and microscopicorientation, and hence to better tensile properties, such as highmodulus and tenacity.

The fibers from each spinneret, upon exiting the coagulation bath aretypically combined to form a single large strand or tow. The fibers arewashed with an appropriate non-solvent, but not necessarily tocoagulant, to remove the desired amount of spinning solvent in one ormore baths or showers and then stretched in one or more draw bathscontaining water that is at or close to boiling. Acidic or basic aqueoussolutions, or acids dissolved in alcohol have been successfully used,although other solutions are expected to be useful. It should be notedthat stretching can also be carried out in conjunction with the washing.

Reference will now be made in detail to the present preferredembodiments of the invention examples of which are illustrated in theaccompanying drawings. Turning now to FIG. 1 a, a scanning electronmicroscope (SEM) micrograph of a 28 μm solid polyaniline fiber spun inaccordance with the teachings of the present invention, shows thatdense, non-porous polyaniline fibers are formed. Voids and pores areundesirable in solid fibers as they can negatively affect the mechanicalproperties of the fibers. After drawing the fiber on the godets, thevoid volume fraction is observed to be <0.001%. FIG. 1 b is a SEMmicrograph of a yarn formed from 20 polyaniline fibers having a twistratio of 7 turns per inch, showing that the fibers have sufficientstructural integrity to be formable into yarns, braids and weaves, asexamples.

Although polyaniline used for the EXAMPLEs hereof was produced frompolymerization of unsubstituted aniline in accordance with the synthesisset forth in Section A, hereinbelow or from other syntheses thereof, theterm polyaniline as used herein includes the following polyanilineswhich are expected to perform in a similar manner. Polyanilines suitablefor use in accordance with the present invention are homopolymers andcopolymers derived from the polymerization of unsubstituted orsubstituted anilines of the form:

where n is an integer between 0 and 2; m is an integer between 3 and 5,such that n+m=5; R₁ is selected so as to be the same or different ateach occurrence and is selected from the group consisting of aryl-,alkyl-, alkenyl-, alkylthio- and alkoxy-moieties having between 1 andabout 30 carbon atoms, cyano-, halo-, acid functional groups, such asthose from sulfonic acid, carboxylic acid, phosphonic acid, phosphoricacid, phosphinic acid, boric acid, sulfinic acid and derivativesthereof, such as salts, esters, and the like; amino-, alkylamino-,dialkylamino-, arylamino-, hydroxyl-, diarylamino-, andalkylarylamino-moieties; or alkyl, aryl, alkenyl, alkylthio or alkoxysubstituted with one or more acid functional groups, such as sulfonicacid, carboxylic acid, phosphonic acid, phosphoric acid, phosphinicacid, boric acid, sulfinic acid and derivatives thereof, such as salts,esters, and the like. R₂ is the same or different at each occurrenceand, is either one of the R₁ substituents or hydrogen.

Polyanilines suitable for use in this invention are generally thosewhich include the following repeat units or a combination thereof havingvarious ratios of these repeat units in the polyaniline backbone:

As an example:

where x represents that fraction of the reduced repeat units in thepolymer backbone and 7 represents at fraction of the oxidized repeatunits in the polymer backbone, such that x+y=1; and z is an integerequal to or greater than about 20. For the polymer in its emeraldinebase oxidation state, x=0.5 and y=0.5.

It is known that the mechanical strength of fibers produced from awet-spinning process is related to the solids content of the spinningsolution. Additionally, higher quality fibers derive from highermolecular weight polyaniline starting material.

High solids content polyaniline solutions prepared for fiber spinningincluded 60 AMPSA molecules per one hundred aniline repeat units in thepolymer backbone since these solutions were found to have the highestconductivity, and are denoted as PANI.AMPSA_(0.6) in what follows (See,for example, “The influence of 2-acrylamido-2-methyl-1-propanesulfonicacid (AMPSA) additive concentration and stretch orientation onelectronic transport in AMPSA-modified polyaniline films prepared froman acid solvent mixture” by M. F. Hundley et al., Synth. Met. 129,291-297 (2002).). Solutions having between 30 and 100 AMPSA moleculesper 100 aniline repeat units can also be processed in accordance withthe teachings of this invention.

The stability of dope solutions against gelation decreases withincreasing solids content and with increasing polyaniline molecularweight. Thus, prior to the present invention, the period of usefulnessof high solids content/high molecular weight solutions has been short.By maintaining lower temperatures during mixing, and adding water to thesolution, gelation of PANI.AMPSA_(0.6)/DCAA solutions can besignificantly retarded. Since the temperature of the solution increasesduring mixing with increasing viscosity due to viscous dissipation ofenergy, solutions were placed in contact with a cooling bath duringpreparation, and the solids were added at a rate sufficiently slow tokeep the solution temperature below about 35° C.

Solutions stored between 0 and 5° C. for 4 weeks were found to producefibers having similar electrical and mechanical properties to those spunimmediately after solution preparation. Solutions stored forapproximately 4 months possessed similar rheological properties to morerecently prepared solutions, and are expected to yield similar qualityfibers, while solutions stored under identical conditions were found tobe partially gelled after about 9 months of storage.

Solutions of PANI.AMPSA_(0.6) in DCAA having between 6 mass % and 14mass % of solids were prepared at 25° C.±5° C. Fiber properties werereproducible given a particular mixing protocol when the mass scale,temperature (±2° C.), and total mixing times were consistent.

The present invention also includes a method for doping, dedoping andredoping polyaniline fibers to obtain chosen properties thereof. Asexamples, the electrochemical activity of polyaniline fibers can begreatly improved, and the selectivity of hollow polyaniline fibers whenused as membranes can be tailored by replacing the dopant present in thefiber as a result of being incorporated into the fiber during theacid-spinning process with a selected anion. As will be demonstratedhereinbelow, dopant substitutions to select one particular property donot necessarily affect other physical properties of the fiber from thegroup of properties including tensile strength, percent extension at thebreak point, and fiber electrical conductivity when fibers are doped tothe same levels. As an example, in order for polyaniline fibers preparedusing AMPSA in the acid-spinning process of the present invention anddescribed hereinbelow to be used for certain electrochemical and otherdevices, the AMPSA is replaced with other dopant acids that arecompatible with the electrolyte, and enable electroactivity of thepolyaniline.

A. Representative Synthesis of High Molecular-Weight, Halogen-FreePolyaniline

Water (6,470 g) was first added to a 50 L jacketed reaction vesselfitted with a mechanical stirrer. Phosphoric acid (15,530 g) was thenadded to the water, with stirring, to give a 60 mass % phosphoric acidsolution. Aniline (1,071 g, 11.5 moles) was added to the reaction vesselover a 1 h period by means of a dropping funnel in the top of thereaction vessel. The stirred aniline phosphate was then cooled to −35.0°C. by passing a cooled 50/50 by mass, methanol/water mixture through thevessel jacket. The oxidant, ammonium persulfate (3,280 g, 14.37 moles)was dissolved in water (5,920 g), and the resulting solution was addedto the cooled, stirred reaction mixture at a constant rate over a 30 hperiod. The temperature of the reaction mixture was maintained at−35.0±1.5° C. during the duration of the reaction to ensure good productreproducibility between batches.

The reactants were typically permitted to react for 46 h, after whichthe polyaniline precipitate was filtered from the reaction mixture andwashed with about 25 L of water. The wet polyaniline filter cake wasthen mixed with a solution of 800 cc of 28% ammonium hydroxide solutionmixed with 20 L of water and stirred for 1 h, after which the pH of thesuspension was 9.4.

The polyaniline slurry was then filtered and the polyaniline filtratewashed 4 times with 10 L of water per wash, followed by a washing with 2L of isopropanol. The resulting polyaniline filter cake was placed inplastic trays and dried in an oven at 35° C. until the water content wasbelow 5 mass %. The recovered mass of dried polyaniline was 974 g (10.7moles) corresponding to a yield of 93.4%. The dried powder was sealed ina plastic bag and stored in a freezer at −18° C. The weight averagemolecular weight (M_(w)) of the powder was found to be 280,000 g·mol⁻¹,although M_(w) values between about 100,000 and about 350,000 g·mol⁻¹have been obtained using this synthesis by controlling the reactiontemperature between 0 and −35° C., respectively. Gas phase chromatograph(GPC) molecular weight data was obtained using a 0.02 mass % solution ofEB in NMP containing 0.02 mass % lithium tetrafluoroborate. The flowrate of the solution was 1 mL·min.⁻¹, and the column temperature was 60°C. The Waters HR5E column utilized was calibrated using Polymer Labs PS1polystyrene standards, and the polymer eluted from the GPC column wasdetected using a Waters 410 refractive index detector coupled with aWaters 996 UV-Vis photodiode array.

The concentration of phosphoric acid was chosen in order to prevent thereaction mixture from freezing at low temperatures. Sulfuric acid,formic acid, acetic acid, difluoroacetic acid, and other inorganic andorganic acids have either been found to be or are expected to besuitable as well. Since the aniline polymerization reaction isexothermic, to ensure good product reproducibility between batches, thetemperature is controlled to keep any exotherm less than a few degrees.

Although this synthesis was used for the polyaniline spinning solutionsset forth hereinbelow, polyaniline can be prepared by any suitablemethod; as examples, chemical polymerization of appropriate monomersfrom aqueous solutions, mixed aqueous and organic solutions, or byelectrochemical polymerization of appropriate monomers in solutions oremulsions.

B. Preparation of Spin Solutions Having 7-14 Mass % PANI.AMPSA_(0.6) inDCAA

Solutions having below 7 mass % of PANI.AMPSA_(0.6) using PANI-EB havinga weight average molecular weight (M_(w)) of ˜300,000 g·mol⁻¹ dissolvedin DCAA were prepared on the 500 g to 1.5 kg scale as described inEXAMPLE 1 hereinbelow. Solutions having between 7 and 14 mass %PANI.AMPSA_(0.6) in DCAA were prepared on the 500 g to 1.5 kg scale inaccordance with the following general procedure.

The chemical compositions of the solutions used in the EXAMPLEShereinbelow are tabulated in TABLE 1. All reported solutions were madeusing PANI-EB having a weight average molecular weight (M_(w)) of˜300,000 g·mol⁻¹. However, fibers have been successfully produced usingpolyaniline having weight average molecular weights between about 90,000and about 350,000 g·mol⁻¹ (defined as high molecular weight polyanilineherein). The use of higher molecular weight polyaniline enables thefibers to survive greater stretch ratios in the spin line withoutbreaking. High stretch ratios are important for obtaining fibers havinghigh electrical conductivity, high modulus and high peak stress.

The PANI-EB powder was dried to achieve the desired individual residualwater contents listed in TABLE 1 under ambient conditions or using avacuum oven at approximately 60° C. The water content of the PANI-EBpowder was determined by thermogravimetric analysis (TGA). If the mass %of water in the PANI-EB powder was found to be lower than the chosenamount, additional deionized water was added to the powder prior topreparing the spin solution to achieve the chosen water content. Thepercentage water in the spinning solutions was between 0.1 and 0.6 mass%, which corresponds to a water content in the polyaniline of between 2and 12 mass %.

Solution A was prepared by first dissolving ½ of the AMPSA (17.4 g) inthe DCAA solvent. The remaining AMPSA (17.4 g) was then ground with thePANI-EB powder forming a PANI/AMPSA powder mixture, and added to theDCAA solution in discrete portions with mixing over a 7 h period.Solutions B, C and D were prepared by dissolving all of the AMPSA in theDCAA, and adding the PANI-EB powder to the DCAA solution in discreteportions with mixing over a 5-7 h period. The total mixing time for eachof these solutions is also listed in TABLE 1.

TABLE 1 Summary of compositions for PANI•AMPSA solutions dissolved inDCAA. Solids % water in % water in Total Max. Content Scale PANI-EB PANIAMPSA DCAA solution mixing temp Label (mass %) (g) (mass %) (g) (g) (g)(mass %) time (h) (° C.) A 12 500 10 27.4 34.8 437.2 0.5 11.5 28 B 12500 10 27.4 34.8 437.2 0.5 12.5 33 C 12 1,500 10 84.0 104.4 1311.6 0.515 28 D 11 1,000 4 46.3 63.7 890 0.2 10 31

For other solutions not reported here, the PANI-EB and AMPSA powderswere combined using a ball mill and added to the DCAA in discreteportions. The final solution properties have been found to beindependent of the method for powder addition, so long as the rate ofpowder addition of each portion was chosen to maintain the solutiontemperature below 35° C. (to avoid gelation).

As the solutions become more concentrated, the viscosity thereofincreases. This results in additional heat being generated by viscousdissipation. In order to minimize heat build-up, coolant was circulatedaround the outside of the mixing vessels employed. The temperature ofthe mixing solution was continuously monitored using a thermocouple toensure that the solution temperature did not exceed 35° C.

To remove entrapped air caused by the mixing process, the solutions weredegassed under vacuum at 50 mbar for 1 h before they were spun intofibers.

Rheological studies of the spinning solutions indicate that theviscosity for 7 solids mass % solutions is between 40 and 60 Pa·s; thatfor 11 solids mass % solutions is between 80 and 120 Pa-s, and that for12 solids mass % solutions is between 120 and 180 Pa·s. Theseviscosities were measured at between 23 and 25° C. at a shear rate ofeither 0.4 s⁻¹ or 0.8 s⁻¹, depending on which rheometer was employed,and were found to vary slightly with the water content of the solutions.

C. Fiber Spinning

In accordance with the teachings of the present invention, twoprocedures are described for coagulating polyaniline spinning solutions:a first method uses ethyl acetate (EA) as the coagulant for the solutionafter it is extruded through the spinneret (EXAMPLE 2), and a secondmethod uses EA as the coagulant followed immersion of the fiber in anacidic solution (for example, phosphoric acid) to further extract DCAAfrom the fiber, and exchange dopants present in the fiber from thespinning process (EXAMPLE 3). Other coagulants and mixtures ofcoagulants (for example, 90 mass % of EA and 10 mass % of acetone wasused as a coagulant) may be used to remove the DCAA from the spinningsolutions. That is, esters, ketones and alcohols, such as butyl acetate,acetone, methylisobutyl ketone, 2-butanone, methanol, ethanol, andisopropyl alcohol in which DCAA is miscible, but in which polyaniline issubstantially insoluble, can be employed to coagulate the solution.Either a wet spinning process in which the spinneret is immersed in thecoagulant, or a dry-wet spinning process, where an air gap is maintainedbetween the spinneret and the coagulant, was successfully employed.

Dope solutions yielding fibers in a continuous spinning process hadviscosities between 8 and 250 Pa-s⁻¹, the upper limit arising fromlimitations of the gear pump utilized.

For both processes, it has been found that applying heat to the fiberassists in the removal of residual DCAA and washing agents (water, as anexample), which has been found to stabilize the fiber properties afterprocessing. By both heating the fiber and stretching it, theconductivity of the spun fiber is increased. For the same stretch ratio,the longer the coagulation time for a fiber, the stronger the resultingfiber becomes. For the same coagulation time, higher fiber stretchingresults in stronger, but more brittle fibers. Otherwise, the fibers spunfrom these two processes possess different properties.

Sample spinning conditions and related properties of the fiber spun intoan EA coagulation bath, and stretched between the two godets whilesimultaneously being heated to between 70 and 100° C., are summarized inTABLE 2. When compared to the dopant-exchanged fibers summarized inTABLE 5, these fibers generally possess higher modulus and peak stress,and higher conductivity.

TABLE 2 Electrical and Mechanical Properties of As-Spun, Heated andStretched, Acid Spun Polyaniline Fibers without Dopant Exchange for10-12 mass % PANI•AMPSA•DCAA Spinning Solutions (150 μm spinneret).Spinning Conditions Residence Time in EA Stretch Ratio and Mechanicaland Electrical (s) Heat Tube Conditions Properties of As-Spun Fibers4-40 1.2-2:1; ~90° C., <10 s Modulus: 1-2 GPa Peak stress: 60-100 MPaExtension at break: 10%-80% Conductivity: 200-550 S/cm 5-40 2-2.4:1;~90° C., <10 s Modulus: 2-5 GPa Peak stress: 100-150 MPa Extension atbreak: 6%-10% Conductivity: 550-1000 S/cm 40-70  1.2-1.8:1; ~90° C.,10-20 s Modulus: 1-2 GPa Peak stress: 60-80 MPa Extension at break:40%-80% Conductivity: 300-600 S/cm 40-70  1.8-2.5:1; ~90° C., 10-20 sModulus: 2-4 GPa Peak stress: 80-150 MPa Extension at break: 6%-40%Conductivity: 600-800 S/cm 70-120 2.5:1; ~90° C., 20-30 s Modulus: 3-5GPa Peak stress: 130-150 MPa Extension at break: 6%-10% Conductivity:750-900 S/cm

It is seen from TABLE 2 that:

(a) When a stretch ratio >2 is applied to the fiber, the flexibility ofthe fiber was reduced, and the extension at break was found to be <10%.

(b) The conductivity of as-spun fiber is greater than 500 S/cm forstretch ratios >2, and less than 500 S/cm for stretch ratios <2.

(c) A 25 s coagulation time is sufficient to coagulate the fiber whenthe spinneret diameter is <150 μm. A greater than 40 s EA bath residenttime combined with a stretch ratio >2.3 has been found to generate highfiber modulus, high peak stress and high conductivity; however, theflexibility of the fiber was reduced, as the extension at break wasfound to be <10%.

Elemental analysis by energy dispersive x-ray spectroscopy (EDS) showsminimal extraction of AMPSA from the fiber during the described spinningprocess. For EDS analysis, the fiber sample is subjected to a highvoltage electron beam that results in X-ray emission having energiescharacteristic of the elements present in the fiber. The amount of DCAAin the fibers was found to depend on the fiber residence time in the EAcoagulation bath. The amount of residual DCAA affects the properties ofaged fiber; that is, DCAA causes diminished fiber mechanical propertiesover time.

Having generally described the invention, the following EXAMPLES provideadditional details thereof.

Example 1 (i) Preparation of Spin Solutions Having 6 Mass %PANI.AMPSA_(0.6) in DCAA

Polyaniline (84.2 g, 0.93 moles of aniline repeat units) and2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA, 115.8 g, 0.56moles) were added to a 2 L plastic vessel containing a ceramic grindingmedia. The contents were milled for 2 h, and 1.0 g of water was added tothe jar contents 30 min. after the milling process was commenced. 60.0 gof the PANI.AMPSA_(0.6) powder was removed from the jar. Dichloroaceticacid (DCAA, 940 g) was placed in a vessel that was maintained at atemperature between 10 and 15° C. to remove heat generated during mixingprocess.

A Silverson SL4RT mixer having a duplex head was immersed in the DCAAand stirred at 1500-2000 rpm. The PANI.AMPSA_(0.6) powder was added withstirring to the DCAA over a 3 h period to produce 1 kg of a 6 mass %solution. The temperature of the stirred solution was kept below 35° C.at all times to prevent gelling. After the powder addition, the solutionwas left to mix for 18 h before being degassed under a dynamic vacuum of˜50 mbar for 1 h.

(ii) Fiber Spinning of Spin Solutions Having 6 Mass % PANI.AMPSA_(0.6)in DCAA

The degassed solution was placed inside of a pressure vessel and 20 psiof nitrogen gas pressure was applied to the vessel to direct thesolution to the gear pump. The solution was passed through a 230 μm porefilter prior to entering the gear pump. The Mahr & Feinpruf gear pumpincluded 2 interlocking cogs which deliver 0.08 cm³ of solution perrevolution. The gear pump was adjusted to deliver 1.3 cm³·min. of thespin solution. The solution was then passed through 230 and 140 μm porefilters before entering a 250 μm diameter spinneret (I/d=4). Thespinneret was immersed in an ethyl acetate coagulation bath (wetspinning). The fiber was passed through the coagulation bath for about 1m before being taken up on a pair of rotating (12.0 rpm; 6.2 m·min.⁻¹.),16.5 cm diameter godet drums immersed in a 1 M solution of phosphoricacid.

The fiber was then passed through a 1.2 m long heat tube maintained at atemperature of 90±10° C. and wound onto a second godet pair having thesame diameter and the first pair, and turning at 15.6 rpm (8.1m·min.⁻¹), thereby stretching the fiber with a 1.3:1 stretch ratio. Thefiber was then collected on a 15 cm diameter bobbin turning at 18 rpm(8.5 m·min.⁻¹) and allowed to dry at ambient conditions for severalweeks. About one month later, a section of the fiber was measured andfound to have a diameter of 56±2 μm, a conductivity of 270±30 S·cm⁻¹, apeak stress of 108±9 MPa, a modulus of 4.1±0.3 GPa, and an extension atbreak of 20±4%.

Example 2 Fiber Spinning Using The A Solution

The fiber spin line included a gear pump and 3, post-pump, in-linefilters (230, 140 and 60 μm pore). Two spinnerets were used: (a) 150 μmdiameter with a length to diameter ratio (I/d) of 4; and (b) 100 μm indiameter with an I/d of 2. The fiber spinning solution was wet spun atambient temperature (between 16 and 25° C.) into a coagulation bath,containing ethyl acetate (EA), although, as stated hereinabove, othercoagulation liquids can be used. The fiber was then wound around a firstpair of 0.165 m diameter godets which were either partially immersed ina chosen solution or simply rotated in air at ambient conditions. Thefiber was subsequently passed through a 1.2 m long heat tube maintainedat a temperature between 50 and 100° C., and wound around a second pairof godets turning between 1.2 and 2.7 times faster than the first godetpair. The second godet drums were not immersed in a solvent. The fiberwas next wound onto a 0.150 m diameter bobbin using a Leesona fiberwinder, and stored for at least 1 d under ambient conditions beforemechanical and conductivity measurements were performed.

Some of the fibers were autoclaved for 2 h and their mechanical andelectronic properties remeasured, then reprotonated (redoped) with 10mass % methanesulfonic acid (MSA) dissolved in methanol. This processwill be set forth in greater detail hereinbelow.

In a representative spin, with a 150 μm diameter spinneret and a pumpflow rate was 0.10 cm³·min.⁻¹, the residence time in the EA bath was 77s. The heating tube temperature was 85° C., and 18.8 g of fiber having adiameter of 86±2 μm were collected over 150 min. The speed of the firstgodet was 3.0 rpm (˜1.56 m·min⁻¹). The speed of rotation of the secondgodet was varied between 1.2 and 2.7 times faster than the first godet,in steps (3.6; 4.5; 5.4; 6.3; 7.2; and 8.1 rpm, as examples). Fibersamples were collected for several minutes at each speed for furthermeasurements (fibers #1-#6). When the stretch ratio was higher than 3.0for these fiber processing conditions, it was found that continuousfiber spinning became difficult, suggesting that the limit of thestretch ratio for these conditions is ≦3 (2^(nd) godet speed of rotation≦9 rpm).

In order to generate fibers having a diameter less than 50 μm, a 100 μmdiameter spinneret was used. The flow rate was reduced to 0.03cm³·min.⁻¹, and the first godet speed was reduced to 2.0 rpm, giving aresidence time of 115 s in the EA bath. The heat tube was kept at 85°C., and the second godet was rotated 2.5 times faster (5.0 rpm) than thefirst godet (fiber #7).

For a flow rate of 0.10 cm³·min.⁻¹ and a first godet speed of rotationof 6.5 rpm, the residence time in EA was 38 s. The heat tube was kept at85° C., and the second godet was rotated 2.5 times faster (16.3 rpm)(fiber #8).

For a flow rate of 0.20 cm³·min.⁻¹ and a first godet speed rotation of13.3 rpm, the residence time in the EA bath was 17.3 s. In order to morecompletely remove the residual solvent in the spun fiber, the heat tubetemperature was raised to 100° C. The stretch ratio was 2.5 (2^(nd)godet speed of rotation of 33.3 rpm). The fibers produced under theseconditions were broke during the spinning process.

TABLE 3 shows the variation of properties with increasing amounts ofstretch between the godets for fibers #1-#6. The correlations betweenstretch ratio and mechanical and electrical properties of tested fibersare shown in FIGS. 1-5.

TABLE 3 Variation of Fiber Properties with Increasing Stretch Ratio.Speed ratio Peak godet 2: Diameter Density Conductivity Stress ModulusExtn. @ godet 1 (μm) (g · cm⁻³), ±5% Denier* (S · cm⁻¹) (MPa) (GPa)break (%) 1.2 93 ± 2 1.68 103 335 ± 25 62 ± 5 0.6 ± 0.1 84 ± 8 (fiber#1) 1.5 84 ± 2 1.59 79 445 ± 40 77 ± 5 1.1 ± 0.1 60 ± 4 (fiber #2) 1.878 ± 2 1.57 68 525 ± 4  70 ± 8 1.4 ± 0.1  42 ± 10 (fiber #3) 2.1 70 ± 21.59 55 630 ± 65 79 ± 2 2.1 ± 0.1  29 ± 10 (fiber #4) 2.4 66 ± 2 1.55 48750 ± 80 97 ± 5 2.6 ± 0.2 14 ± 4 (fiber #5) 2.7 62 ± 2 1.60 43  810 ±100 111 ± 3  2.9 ± 0.4 12 ± 4 (fiber #6) *Denier is defined as thenumber of grams per 9000 m of fiber.

From TABLE 3 it may be observed that the diameter and the percentextension at break are seen to decrease proportionally with increasingstretch ratio which is suggestive that the fiber chains are increasinglyaligned as the stretch ratio increases. See FIG. 2 and FIG. 3 hereof. Asthe stretch ratio increases, the denier decreases; however, the density(averaging 1.60±0.08 g·cm⁻³) does not change within experimental error(within 5%, TABLE 3). The peak stress, modulus and room temperatureelectrical conductivity of the fibers all increase with increasingstretch ratio, also indicating increasing polymer chain alignment. SeeFIG. 4, FIG. 5 and FIG. 6 hereof. The stress/strain curves for typicalfiber samples for each stretch ratio (fibers #1-#6) are shown in FIG. 7hereof.

Spinning was also performed using a 100 μm diameter spinneret at severalflow rates. These fibers were stretched 2.5 times between the godets,and the resulting properties are set forth in TABLE 4 (fibers 7-9).

TABLE 4 Variation of Fiber Properties with Flow Rate. Temp heat 1stgodet Peak Pump rate tube speed Diameter Density Conductivity StressModulus Extn. @ (cm³ · min⁻¹) (° C.) (rpm) (μm) (g · cm⁻³) Denier (S ·cm⁻¹) (MPa) (GPa) break (%)  0.03 85 2.0 44 ± 2 1.49 20 790 ± 110 142 ±6 4.1 ± 0.3 8.0 ± 1.3 (fiber #7) 0.1 85 6.5 45 ± 2 1.47 21 790 ± 110 130± 3 3.2 ± 0.8 8.9 ± 3.6 (fiber #8) 0.2 100 13.3 43 ± 2 1.57 21 960 ± 110149 ± 4 4.5 ± 0.6 5.7 ± 0.5 (fiber #9)

The fiber residence times for these three runs (from top to bottom)decrease from 115 s to 36 s, and finally to 17 s. There is littledifference between these fibers and fiber #6 (TABLE 3), although thediameter of these samples is smaller than that of fiber #6 because asmaller diameter spinneret was used. These fibers are observed to haveslightly higher peak stress, and modulus, while the percent extension atbreak is lower. The conductivities are similar to that for fiber #6.Note that the fiber diameters for fibers #7-#9 are between 43 and 45% ofthe spinneret diameter, similarly to those for fiber #5 and fiber #6from TABLE 3 (stretched at 2.4 and 2.7 times, respectively) which arebetween 44 and 41% of the spinneret diameter respectively. However, thedensities of fiber #7 and fiber #8, spun at 0.03 and 0.10 cm³·min.⁻¹,are slightly lower than that for fiber #9, which is similar to thevalues in TABLE 3. Fiber #9 of TABLE 4 is seen to have the highestconductivity which may be due to additional fiber stretching in thecoagulation bath.

D. Dopant Manipulation

(1) Spin Line Processing:

Since the performance of conducting-polymer-based devices is known to bedependent on the properties of dopant anions, for certain applicationsit may be desirable to replace the AMPSA and DCAA present in the fibersas a result of the acid-spinning process with other dopants. Dopantspresent in polyaniline fibers can be partially or totally replaced withselected dopants during: (a) the spinning process; (b) post-spinningdopant manipulation; and (c) fiber conversion to the insulating EB form(dedoped) as part of the spinning process, followed by redoping with theselected acid. The three approaches generated the same electrical andmechanical properties in the doped form of the fibers.

As stated hereinabove, removal of DCAA from the as-spun fiber isadvantageous since residual DCAA has been found to slowly degrade themechanical properties of polyaniline fiber, and because it is ahazardous compound.

As an example of a dopant-exchange process for polyaniline fibersprepared from a 12 mass % solution of PANI.AMPSA dissolved in DCAA, thefirst godet bath was filled with an aqueous solution of 1 M phosphoricacid. Phosphoric acid was chosen because it imparts good thermalstability to the doped fiber, and is a relatively inexpensive acid. Aresidence time of 1 min. in the dopant exchange solution (at 1 M H₃PO₄)was found to be sufficient to replace AMPSA with phosphoric acid. Otheracids are expected to be suitable for replacing the AMPSA and DCAAmolecules in the as-spun polyaniline fiber in a similar manner.

The residence time in the coagulation bath was chosen such that acidshaving low pKa values will replace the AMPSA dopant molecules in thefiber. By contrast, the majority of DCAA molecules present in the fiberfrom the spinning solution were found to be removed in the ethyl acetatecoagulation bath (note that AMPSA has minimal solubility in EA). TABLE 5provides a summary of spinning conditions and the resulting mechanicaland electrical properties of fibers after dopant exchange usingphosphoric acid.

TABLE 5 Electrical and Mechanical Properties of As-Spun, Heated,Stretched and Washed, Acid Spun Polyaniline Fibers without DopantExchange for 10-12 mass % PANI•AMPSA•DCAA Spinning Solutions (~150 μmspinneret). Spinning Conditions Residence Time in Stretch Residence 1MH₃PO₄ Ratio and Mechanical Time in Aqueous Heat Tube and ElectricalProperties EA (s) Solution (s) Conditions of As-Spun Fibers 20-25 20-401.1-1.5:1; Modulus: 1.5-2.5 GPa ~90° C., Peak stress: 60-100 MPa 5-10 sExtension at break: 20%-60% Conductivity: 200-350 S/cm  6-25 40-601.1-1.3:1; Modulus: 1-1.5 GPa ~90° C., Peak stress: 60-100 MPa 5-10 sExtension at break: 40%-60% Conductivity: 200-400 S/cm

The results for the composition of as-spun polyaniline fibers that havebeen processed with and without a H₃PO₄ dopant exchange step aresummarized in TABLE 6. The 56 s residence time in the first godet bathcontaining the phosphoric acid solution resulted in substantially all ofthe AMPSA and DCAA in the solid fiber being replaced with phosphoricacid. The dopant exchanged fiber was found to have similar electricaland mechanical properties to the control fiber.

The stretch ratio for these dopant exchanged fibers was kept below 1.5during the process, since higher stretching ratios result in the fiberbreaking between the two godet baths. This is likely the result of theremoval of the AMPSA which acts as a plasticizer for polyaniline fibers.Generally, when a phosphoric acid dopant exchange step is introducedafter the coagulation bath, the as-spun fibers are more flexible thanthe unexchanged fibers of TABLE 2.

Example 3 Dopant Exchange During Fiber Spinning

As an example of how dopant exchange procedure affects the composition,electrical and mechanical properties of doped polyaniline fibersproduced under the same spinning conditions, fiber was spun fromsolution B at a rate of 0.60 cm³·min⁻¹ (0.76 g·min⁻¹) into an EAcoagulation bath with a coagulation length of 4.6 m which corresponds toa residence time of 38 s. The first godet pair was rotated 15.0 rpm andthe second godet pair was rotated 1.2 times faster. The temperature ofthe 1.2 m heat tube placed between the godet baths was maintained at 90°C. The first godet was immersed in a 1 M aqueous phosphoric acidsolution in order to exchange dopants during the spin process. Theresidence time of the fiber in the first godet bath was 56 s. Thecomposition of both as-spun polyaniline fibers were assessed using EDS.With the use of an empty first godet bath, control fibers were spununder the same conditions without dopant exchange.

TABLE 6 Mechanical and Electrical Properties, and Composition ofPolyaniline Fibers that have been Processed with and without a H₃PO₄Dopant Exchange Step. Peak Extn. @ Dopant Diameter Conductivity StressModulus break Exchange (μm) (S · cm⁻¹) (MPa) (GPa) (%) Composition No 72± 2 300 ± 10 55 ± 2 1.6 ± 0.3 20 ± 15 PANI•AMPSA_(0.58)•DCAA_(0.18) Yes70 ± 4 315 ± 40 69 ± 2 2.1 ± 0.2 26 ± 5  PANI•H₃PO_(4 0.70)AMPSA_(0.02)•DCAA_(0.02)

Residence times in the EA coagulation bath exceeding 38 s weresubsequently found to produce weaker fibers. Residence times in the EAcoagulation bath between 6 and 25 s gave a good combination ofelectrical and mechanical properties when a H₃PO₄ dopant exchange stepis employed.

Example 4 Dedoping During Fiber Spinning

In another embodiment of the present invention for removing acid dopantsfrom spun fibers (AMPSA and DCAA), the polyaniline was dedoped into itsEB oxidation state. This was achieved by immersing the first godet in anammonium hydroxide solution (317 g of 30% NH₄OH in 22 L of water) havinga pH of 10.8. To spin the fiber, the C spin solution was directedthrough a twin-hole spinneret (150 μm diameter) with a gear pump flowrate of 0.60 cm³·min⁻¹. The residence time in the coagulation bath was56 s, with the first godet being rotated at 10.5 rpm. In order to verifythe effect of base washing time on the mechanical properties, theresidence time of the fiber in the ammonium hydroxide was varied. Afirst fiber sample was collected with the fiber undergoing 20 turnsaround the first godets, a second fiber underwent 5 loops, and a thirdsample made a single loop through the first godet bath. The heat tubewas set to 80° C. and the second godet was rotated 1.2 times faster(12.6 rpm) than the first godet, and the fiber was collected on abobbin. The effect of residence time in the first godet bath on thefiber properties is summarized in TABLE 7.

TABLE 7 Effect of Residence Time in a pH 10.8 First Godet Bath on theComposition and Electrical Properties of the Resulting DedopedPolyaniline Fiber. Residence Peak time in first Diameter Stress ModulusExtn. @ godet (s) (μm) (MPa) (GPa) Break (%) Composition 60 52 ± 2 204 ±7 7.6 ± 0.3 3.2 ± 0.2 PANI•AMPSA_(0.02)•DCAA_(0.01) 15 60 ± 2 167 ± 65.7 ± 1.2 8.2 ± 5.4 PANI•AMPSA_(0.08)•DCAA_(0.05) 3 84 ± 2  43 ± 3 1.3 ±0.2 51 ± 27 PANI•AMPSA_(0.20)•DCAA_(0.10)

Based on EDS analysis of the as-spun fibers, a 3 s residence time in thefirst godet was found to be sufficient to remove approximately 90% ofthe DCAA and 65% of the AMPSA from the fiber. The fiber entering thefirst godet bath typically has a composition ofPANI•AMPSA_(0.58)·DCAA_(0.91). However, due to DCAA and coagulant EAtrapped in the as-spun fiber, the fiber was soft and weak. As moresolvent and coagulant were removed, the fiber was found to have a highermodulus and became more brittle. The conductivity of the fibers thatresided in the first godet bath between 15 s and 60 s indicates thatthey were essentially dedoped, as their resistance was greater than 20MΩ. The fiber spun with a residence time of 3 s in the first godet bath,has a conductivity of 10±4 S·cm⁻¹.

Example 5 (2) Post-Spin Line Treatment of Fibers (i) SteamDedoping/Redoping

Fiber #9, processed in accordance with EXAMPLE 2 (TABLE 4), whichdemonstrated high values for peak stress, modulus, and room temperatureconductivity, was next dedoped with steam at 20 psi for 2 h (fiber #10)and then reprotonated by soaking in 10% methanesulfonic acid (MSA) inmethanol for 19 h (fiber #11). The as-spun fiber was also washed inwater for 20 h as a control for the steam dedoped material (TABLE 8).

TABLE 8 Fiber #9 Properties after Steam Dedoping (fiber #10), Redoping(fiber #11), or Immersion in Water (fiber #12). Post Diameter DensityConductivity Peak Stress Modulus Extn. @ treatment (μm) (g · cm⁻³)Denier (S · cm⁻¹) (MPa) (GPa) break (%) As-spun 43 ± 2 1.57 21  960 ±110 142 ± 6  4.1 ± 0.3 8.0 ± 1.3 (fiber #9) Steam 31 ± 2 1.24 8.4 160 ±40 663 ± 53 21 ± 5  5.4 ± 3.5 dedoped (fiber #10) Redoped 36 ± 2 1.65 151070 ± 180 236 ± 15 7.7 ± 1.5 5.4 ± 1.2 (fiber #11) Immersed 40 ± 2 0.9511  0.4 ± 0.2 279 ± 27 6.8 ± 0.4 12 ± 2  In Water (fiber #12)

TABLE 8 shows that the steam-dedoped fiber has the largest peak stressand modulus among the polyaniline fiber measured. This peak stress isapproximately 6 gpd (grams per denier), and the modulus is about 190gpd. By contrast, the strongest polyacrylonitrile (PAN) fibers producedby gel spinning PAN having a M_(w) value of 500,000 g·mol⁻¹ has a peakstress of 7-9 gpd and a modulus of 100-125 gpd, with about 7% extensionat break. The MSA-protonated polyaniline fiber conductivities are higherthan those for the as-spun AMPSA doped fibers (1070 vs. 960 S·cm⁻¹),with improved peak stress and modulus values when compared to theas-spun AMPSA doped fiber.

The temperature dependence on the dc conductivity provides usefulinformation about the microscopic structure of the materials.PANI.AMPSA_(0.6) fibers are a highly conductive system with values ofthe conductivity varying from 50 S·cm⁻¹ for unstretched materials to1200 S·cm⁻¹ for stretch, oriented materials. Samples were mounted in thecontrolled environment of a cryostat. The cryostat was evacuated toapproximately 10⁻⁴ Torr, and the temperature inside the chamber wasbrought to 4K using liquid helium. Temperature was controlled to within0.5% as the temperature was raised to 350K from 4 K. Conductivity(inverse of volume resistivity) as a function of temperature for fibersamples was measured using ASTM procedures (Designation No. D4496-87).

FIG. 7 is a graph of the temperature dependence of the conductivity foras-spun, steam dedoped, and fibers redoped with MSA (fibers #9, 10 and11, respectively). The conductivity of the autoclaved fiber is seen todrop by five orders of magnitude at 4 K, while subsequent redoping ofthe fiber substantially restored the fiber conductivity, although withan altered conductivity profile.

Example 6 (ii) Dedoping of Acid-Spun Solid and Hollow Fibers

(a) Solid Fibers:

A dedoping procedure for removing dopants (AMPSA and DCAA, as examples)from the as-spun fibers which results in fibers having lowroom-temperature conductivity is now described. Essentially all of thedopants are removed from the as-spun fiber without substantiallychanging the mechanical properties of the fiber. Fibers were dedoped byplacing them in contact with de-ionized water or with a 0.1 M solutionof NH₄OH, or by exposing the fibers to steam at 15 psi in an autoclave.

AMPSA-doped polyaniline fibers (70 μm) were spun from the 6 mass %polyaniline/AMPSA/DCAA solutions described in EXAMPLE 1, hereinabove.For polyaniline fibers to be dedoped using water or a basic solution, 1m lengths of fiber were submerged in 200 mL of either de-ionized wateror 0.1 M NH₄OH aqueous solution for between 15 min. and 3 h. The highvolatility of ammonium hydroxide compared with sodium hydroxide permitsits complete removal from the fiber; it was found that use of sodiumhydroxide results in the incorporation of sodium ions into the resultingfiber. For the steam-processed fibers, substantial fiber dedopingrequired that the fibers be exposed to approximately 15 psi of steam forbetween 1 and 8 h. The fibers were then dried overnight under ambientconditions before the room temperature conductivity, elemental analysisand tensile properties of the dedoped fibers were measured.

The effectiveness of the dedoping method was determined by measuring theroom temperature conductivity of the polyaniline fiber. The conductivityof the as-spun fiber was 390 S·cm⁻¹. TABLE 9 summarizes the roomtemperature conductivity for the water and base soaked fibers, and thefibers exposed to steam at 15 psi for different processing times. Theresistance of the base-soaked fibers was >30 MΩ.

TABLE 9 Conductivity of Polyaniline Fiber after Dedoping. Conductivity(S · cm⁻¹) Dedoping Fiber Exposed Fiber Fiber Exposed Time to WaterExposed to 0.1M NH₄OH to Steam 15 min. 0.21 <4 × 10⁻⁴ X 30 min. 0.17 <4× 10⁻⁴ X 1 h 7.5 × 10⁻² <4 × 10⁻⁴ 38 2 h 5.5 × 10⁻² <4 × 10⁻⁴ 1.9 3 h9.8 × 10⁻³ <4 × 10⁻⁴ 0.25 5 h X X 0.22 8 h X X 0.12 X - experiment notperformed.

The results of TABLE 9 show that for any given dedoping time, fibersexposed to the 0.1 M aqueous NH₄OH solution display the lowestconductivity. The conductivity of the base-dedoped fiber dropped rapidlywhen compared with that for the fibers dedoped in water or by steam.Dedoping the polyaniline fibers using the autoclave procedure is a slowprocess for extracting the dopants from the fiber. Dedoping treatmentsalso caused the fiber diameter to decrease from 70 μm for the as-spunfiber to 50 μm for the fiber having the lowest conductivity. The rate ofchange of the fiber diameter follows the loss of conductivity, with thediameter decreasing rapidly for the base-dedoped fibers compared withthe decrease for the autoclaved fibers.

EDS spectra of the as-spun and dedoped fibers recorded at a beamacceleration voltage of 10 kV revealed that carbon, nitrogen, oxygen,phosphorus, sulfur and chlorine were present in the fiber. Hydrogenatoms cannot be detected by this technique. The dedoped fibers wereoriginally spun into an EA coagulation bath and the first godet wasimmersed in a 1.0 M aqueous solution of phosphoric acid. This resultedin the partial extraction of AMPSA molecules from the doped polyanilinefiber since AMPSA is highly soluble in aqueous solutions, and in thereplacement of the AMPSA with phosphoric acid molecules.

The EDS spectra of the as-spun fiber showed its composition to bePANI.AMPSA 0.18 DCAA_(0.65) H₃PO_(4 0.11). The mole % of key elementsfound in dopant molecules as determined by EDS analysis of the dedopedfibers is shown in TABLE 10 to TABLE 12 for the fibers exposed to water,the fibers exposed to base, and the fibers exposed to steam at 15 psi,respectively.

TABLE 10 Composition of Elements Found in the Dopant Molecules forPolyaniline Fibers Dedoped using Water. Time of Mol % Sulfur in Mol %Phosphorus in Mol % Chlorine Dedoping fiber fiber in fiber 15 min. 0.730.19 2.6 30 min. 0.74 0.16 2.4 1 h 0.66 0.12 2.2 2 h 0.66 0.11 1.7 3 h0.64 0.10 1.6 (36% AMPSA (84% H₃PO₄ removed) (77% DCAA removed) removed)Final Fiber Composition PANI•AMPSA_(0.05) DCAA_(0.07) H₃PO_(4 0.01)

TABLE 11 Composition of Elements Found in the Dopant Molecules forPolyaniline Fibers Dedoped Using 0.1M NH₄OH. Time of Mol % Sulfur in Mol% Phosphorus in Mol % Chlorine Dedoping fiber fiber in fiber 15 min.0.52 0.04 0.34 30 min. 0.37 0.05 0.17 1 h 0.39 0.04 0.16 2 h 0.31 0.050.14 3 h 0.31 0.04 0.15 (69% AMPSA (94% H₃PO₄ removed) (98% DCAAremoved) removed) Final Fiber Composition PANI•AMPSA_(0.02) DCAA_(0.01)

TABLE 12 Composition of Elements Found in the Dopant Molecules forPolyaniline Fibers Dedoped Using Steam. Time of Mol % Sulfur in Mol %Phosphorus in Mol % Chlorine Dedoping fiber fiber in fiber 1 h 0.80 0.081.8 2 h 0.82 0.03 1.2 3 h 0.80 0.04 1.0 5 h 0.68 0.05 0.79 8 h 0.58 0.070.29 (42% AMPSA (89% H₃PO₄ removed) (96% DCAA removed) removed) FinalFiber Composition PANI•AMPSA_(0.04) DCAA_(0.01)

The above results show that the dedoping methods employed are effectivefor removing the DCAA and phosphoric acid from the as-spun fiber.However, not all of the AMPSA molecules are extracted from the fiber.The fibers immersed in a 0.1 M aqueous bath containing NH₄OH showed thelowest sulfur content of the three methods for a given dedoping time.Additionally, the fiber immersed in base for 15 min. possessed lowersulfur content than the fiber immersed in water for 3 h, and the fibersteam treated for 8 h. Comparing the sulfur content for the dedopedfibers to their conductivity values listed in TABLE 9 indicates that thehigher conductivity for fibers exposed to water and steam arises from ahigher residual AMPSA doping level in these fibers. It is believed bythe present inventors that the residual AMPSA (3-5% of the AMPSA fromthe original spinning solution) may have been cross-linked during thefiber spinning process and, therefore, resisted being extracted by thesededoping procedures.

The tensile properties of some of the dedoped fibers listed in TABLE 8were measured to determine how dedoping procedures affect the mechanicalproperties of the fibers. Fibers were stretched at 10 mm·min.⁻¹ using a1 lb load cell. The results are summarized in TABLE 13, in addition tothe tensile properties of the as-spun fiber. The dedoped fibers listedin TABLE 13 display improved tensile strength and modulus when comparedto the as-spun fiber, but are substantially more brittle. While thefibers soaked in base and in de-ionized water showed similar mechanicalproperties, these properties were inferior to those of the autoclavedfibers. Moreover, longer exposure time to the steam results in fibershaving a higher modulus; however, the fiber becomes more brittle. It isbelieved by the present inventors that this is due to the more completeremoval of the AMPSA and DCAA dopant molecules from the fiber, which areknown plasticizers for polyaniline.

TABLE 13 Mechanical Properties of the Dedoped Fibers. Peak Modulus FiberStress (MPa) (GPa) Extn. @ Break (%) As-spun 170 5.8 19 Dedoped Fibers 2h steam exposure 430 11.0 12 8 h steam exposure 420 12.3 8 30 min. in0.1M NH₄OH 280 9.5 4 30 min. in water 250 9.0 4

In summary, immersing the fibers in a 0.1 M aqueous solution of NH₄OHproduced fibers having the lowest conductivity and the lowestconcentration of the as-spun dopants in the dedoped fiber. Additionally,this process is substantially faster than exposing the fibers to eitherde-ionized water to steam at 15 psi. Exposing the fibers to base beyond30 min. was observed to have only a small effect on the elementalcomposition of the fiber. An advantage of exposing the as-spun fibers tosteam is that this procedure produces dedoped fibers with superiormechanical properties than fibers dedoped by immersion. The steamdedoped fibers showed the slowest rate of removal of AMPSA from thefiber.

(b) Hollow Fibers:

Doped, hollow polyaniline fibers were spun using a 55-28-16 (0.055in.×0.028 in.×0.016 in.) spinneret. The spinneret included twoconcentric cylinders having openings at both ends. The polyanilinespinning solution was extruded through the gap between the outerdiameter of the inner cylinder and the inner diameter of the outercylinder, while a bore fluid was simultaneously pumped through theinside of the inner cylinder, thereby generating the hollow fiber. Forthe 55-28-16 spinneret, the diameter of the exit hole of the outercylinder was 0.055 in., the outer diameter of the exit hole of the innercylinder was 0.028 in., and the inner diameter of the exit hole of theinner cylinder was 0.016 in. The spinneret had an I/d ratio of 1. Thedimensions of this spinneret and its I/d ratio, and the spinneretdimensions and I/d ratios set forth hereinabove should not be consideredas a limitation on the scope of the present invention. Solution D wasdelivered to the spinneret after passing through 3 in-line filters (230,140, and 90 μm pores) at a flow rate of 1.5 cc·min⁻¹ using a gear pump,while acetone bore fluid was delivered to the spinneret at a flow rateof 0.5 cc·min⁻¹ using a pump. Any of the coagulants listed in Section Chereof can be used as a bore fluid. An ethyl acetate coagulation bathwas used to precipitate the polyaniline solution. After leaving thecoagulation bath, the hollow fiber was directed around a pair of 0.165 mdiameter godets drums rotating at 2 rpm. The fiber then passed through a1.2 m long heat tube maintained at 70° C., as heating the fiber enablesthe fiber to be stretched by a second pair of godets rotating 1.2 timesfaster than the first godet. Neither set of godets was immersed in asolvent. The fiber was subsequently wound onto a 0.150 m diameter bobbinof a Leesona fiber winder.

Hollow fibers were contacted with a 0.1 M solution of ammonium hydroxidefor 1 h for dedoping. However, due to the larger wall thickness of theacid-processed hollow fibers (˜200 μm), a longer dedoping time may bemore effective. Approximately 100 mL of the ammonium hydroxide solutionfor every gram of the as-spun hollow fiber was used. The hollow fiberswere then soaked in deionized water for 1 h, which assisted in removingthe ammonium hydroxide solution from the hollow fiber. Fiber #19 inTABLE 14 was subsequently contacted with methanol for 18 h after beingdedoped with the ammonium hydroxide solution.

After immersing the hollow fibers in the ammonium hydroxide solution for1 h, the color of the fiber was observed to change from dark blue tobronze. The effect of different dedoping methods on the composition ofthe dedoped fiber was also assessed using energy dispersive X-rayspectroscopy (EDS). The results for the EDS analysis and resistancemeasurements for seven replicate dedoped acid-processed hollow fibersare summarized in TABLE 14. The DCAA content for all of these dedopedhollow fibers was at the level of the background noise, while AMPSA wasfound in several of the hollow fibers, especially fibers #14-#16 ofTABLE 14. The high AMPSA content in these fibers is likely responsiblefor the lower resistance values measured for these fibers. Additionally,scanning electron microscope examination of the cross sections of thesefibers indicate that the bore of the hollow fiber collapsed, therebymaking it difficult for the ammonium hydroxide solution to penetrate theinside of the fiber.

TABLE 14 EDS Analysis and Resistance Data for Acid-Processed HollowFibers Dedoped Using Ammonium Hydroxide. Mole ratio of Mole ratio ofResistance/Length Dedoped AMPSA molecules DCAA molecules of fiber Fiber# to N atoms in PANI to N atoms in PANI (Ω · cm⁻¹) 13 0.03 0.00 >20 MΩ14 0.06 0.01 14 MΩ 15 0.08 0.01 7 MΩ 16 0.06 0.01 8 MΩ 17 0.00 0.01 >20MΩ 18 0.01 0.00 >20 MΩ 19 0.00 0.00 >20 MΩ

Example 7 (iii) Redoping of Solid and Hollow Fibers

(a) Redoping Solid Fibers with Triflic Acid:

To achieve minimal AMPSA and DCAA content, high room-temperatureconductivity, good mechanical properties, and high electroactivity andelectrochemical actuation in organic and ionic liquid electrolytes,dedoped fibers were redoped with triflic acid due to its small ionicradius and its high solubility in organic solvents. The enhancedelectroactivity of the triflic acid electrochemically redoped fiber canbe clearly seen in the order of magnitude increase in the cyclicvoltammogram of the fiber shown in FIG. 9( a), when compared with thecyclic voltammogram shown in FIG. 9( b) for the as-spun, AMPSA-dopedfiber.

One meter lengths of polyaniline fiber were first dedoped for 30 min.using a 0.1 M aqueous solution of NH₄OH as described EXAMPLE 6ahereinabove. The dedoped polyaniline fibers were subsequently immersedin 200 mL of either a 0.1 M or a 1.0 M triflic acid solution for achosen period of time. The redoped fibers were dried overnight underatmospheric conditions before their room temperature conductivity,elemental analysis and tensile properties were measured.

The room temperature conductivity, mechanical properties and fibercomposition of the polyaniline fibers redoped with triflic acid underdifferent conditions are summarized in TABLE 15. Both the conductivityand fiber composition are seen to be influenced by the redoping time andthe triflic acid concentration. As the concentration of the triflic acidis increased from 0.1 M to 1.0 M, the redoping time required to achievethe maximum observed conductivity is decreased from 24 h to 16 h. Themaximum conductivity obtained for the redoped fiber is lower than thatfor the as-spun fiber (310 S·cm⁻¹ v. 390 S·cm⁻¹). Longer doping timesresulted in more triflic acid being incorporated into the fiber asdetermined by EDS analysis at a beam acceleration voltage of 10 kV,which explains the increase in the conductivity.

The fiber diameter increased from 50 μm for the dedoped fiber to 62 μmfor the redoped fiber. This diameter is lower than the diameter of theas-spun fiber (70 μm), which reflects the smaller volume of the triflateanion compared to the AMPSA anion. From the data in TABLE 15, therelationship between the doping level, as determined by EDS analysis,and the room temperature conductivity of the fiber is plotted in FIG. 10hereof. The conductivity increases with doping level, and levels offafter the doping level exceeds 0.45. This is consistent with thepolyaniline becoming fully doped; the theoretical limit for fully dopedpolyaniline is a doping level of 0.5.

The redoped fibers showed essentially identical or slightly improvedpeak stress and modulus to the as-spun (170 MPa and 5.8 GPa,respectively). The redoped fibers also showed an extension at break thatwas twice that for the as-spun fiber (19%). The fiber doped in 1.0 Mtriflic acid for 15 min. was observed to have the lowest doping level.It is believed by the present inventors that the triflic anion acts as aplasticizer for the doped polyaniline fiber.

TABLE 15 Mechanical Properties of Polyaniline Fiber Redoped with TriflicAcid. Extn. @ Conductivity Peak Stress Modulus Break Fiber (S · cm⁻¹)Fiber Composition (MPa) (GPa) (%) As-spun 390 PANI•AMPSA_(0.18) 170 5.819 DCAA_(0.65)H₃PO_(4 0.11) 1.0M CF₃SO₃H 110 PANI•Triflate_(0.17) 1605.2 23 15 min. 1.0M CF₃SO₃H 170 PANI•Triflate_(0.23) 180 5.2 39 30 min.1.0M CF₃SO₃H 260 PANI•Triflate_(0.34) 170 5.2 44 1 h 1.0M CF₃SO₃H 270PANI•Triflate_(0.39) 160 5.0 47 2 h 1.0M CF₃SO₃H 280PANI•Triflate_(0.38) 170 5.2 42 4 h 1.0M CF₃SO₃H 280PANI•Triflate_(0.38) 180 5.5 35 8 h 1.0M CF₃SO₃H 310PANI•Triflate_(0.44) 180 5.9 35 16 h 1.0M CF₃SO₃H 310PANI•Triflate_(0.55) 190 5.7 47 24 h 0.1M CF₃SO₃H 190PANI•Triflate_(0.25) 200 6.5 36 8 h 0.1M CF₃SO₃H 240PANI•Triflate_(0.33) 190 5.7 35 16 h 0.1M CF₃SO₃H 300PANI•Triflate_(0.50) 200 5.8 40 24 h

The following observations can be made with respect to the effect ofredoping conditions on the electroactivity of the PANI.CF₃SO₃ fibers:

-   -   (a) Conductivity of the fiber is an important factor in        determining its electroactivity and actuation in organic and        ionic liquid electrolytes. Low electroactivity and no actuation        was observed for the least conductive fiber (16 S·cm⁻¹);    -   (b) For conductivities between 110 and 260 S·cm⁻¹ (redoping time        longer than 15 min. in 1.0 M CF₃SO₃H), enhanced fiber        electroactivity was obtained; and    -   (c) For fibers having conductivity greater than 260 S-cm⁻¹        (redoping time longer than 1 h in 1 M CF₃SO₃H), the        electroactivity of the fiber became independent of the fiber's        conductivity.

(b) Redoping Solid Fibers with Other Acids:

In a similar manner to the redoping of solid fibers with triflic acid,solid fibers were also redoped with the acids listed in TABLE 16. Exceptfor oxalic acid which is solid, these acids are liquids at roomtemperature. Solid polyaniline fibers were spun in accordance with themethod described in EXAMPLE 2 hereinabove, and were dedoped by contactwith a 0.1 M ammonium hydroxide aqueous solution for 30 min. The fiberswere then redoped using 1.0 M aqueous of the acids listed in TABLE 17for 16 h.

Electrical and mechanical properties for the redoped fibers are alsosummarized in TABLE 16, which also includes the data for the as-spunpolyaniline fiber, and for unredoped fiber dedoped using ammoniumhydroxide. It is seen that dedoping the fiber caused the modulus andpeak stress to be higher than the as-spun polyaniline fiber, but thefiber was more brittle. Upon redoping the fiber, the lower the pKa ofthe redoping acid, the more conductive the fiber becomes. There appearsto be no correlation between the pKa of the acid and the mechanicalproperties of the redoped fiber. All of the redoped fibers possessed ahigher percent extension at break than the EB fiber, but had lowermodulus and peak stress. The fiber redoped with HCl showed the highestmodulus, while the fiber redoped with MSA possessed the highest percentextension at break of the measured redoped fibers. Acrylic acid producedthe fiber with the highest tensile strength of the measured redopedfibers.

TABLE 16 Electrical and Mechanical Properties of Polyaniline Fiber afterAcid Redoping. Modulus Peak Stress Extn. @ Conductivity Fiber pK_(a)(GPa) (MPa) Break (%) (S · cm⁻¹) As-spun 4.1 ± 0.3 154 ± 5  10.9 ± 1.5424 ± 22 EB fiber 7.6 ± 0.2 187 ± 9   3.0 ± 0.4 <4 × 10⁻⁴ (dedoped)Redoping Acid HCl −2.2 5.6 ± 0.7 156 ± 23  3.3 ± 1.0 383 ± 17 MSA −2.02.3 ± 0.2 104 ± 6  32.5 ± 5.6 333 ± 13 Oxalic Acid 1.23 2.8 ± 0.5 106 ±10  8.2 ± 2.9 188 ± 12 Pyruvic Acid 2.39 3.3 ± 0.1 107 ± 11 15.0 ± 1.4106 ± 21 Acrylic Acid 4.25 4.7 ± 0.9 179 ± 11 23.6 ± 4.8 45 ± 9

(c) Redoping of Hollow Fibers:

Hollow fibers were similarly redoped using acids having smaller volumethan AMPSA (volume=169 Å³), and having different values for their aciddissociation constants. The acid-processed hollow fibers were firstdedoped using NH₄OH according to the procedure set forth in sectionEXAMPLE 6b hereinabove. The fiber was then divided into five 1 m lengthsand each length was contacted with a 100 mL portion of a 1.0 M aqueoussolution of one of the acids listed in TABLE 17 for 16 h. The redopedfibers were then allowed to dry under ambient conditions before beingcharacterized.

After drying under ambient conditions, the fibers redoped with MSA orHCl became brittle and difficult to handle without damaging the fiber.However, the fibers that were redoped with the other acids were moreflexible. The brittleness of the fibers doped with HCl and MSA arereflected scanning electron microscope cross-sections of these fiberswhich showed cracks caused from fiber fracturing at room temperature.Analysis of the cross sections of fibers redoped with oxalic acid,pyruvic acid and acrylic acid reveal that the integrity of the hollowfibers remained unaffected by the dedoping/redoping process. It isbelieved by the present inventors that acids having strong dissociationconstants (see TABLE 16 hereof) cause the hollow fibers to becomebrittle upon redoping.

Analysis of cross sections of the redoped hollow fibers also indicatedthat the fibers expanded upon redoping. The fiber redoped with HClshowed the smallest degree of expansion (˜4%), while the largest degreeof expansion was observed for the fiber redoped with oxalic acid (˜9%).This effect correlates with the ionic radius of these anions, sincefiber swelling is caused by insertion of anions into the polymer as thepolymer is doped in order to balance the positive charge formed on thepolymer backbone.

Resistivity measurements for the redoped hollow fibers are summarized inTABLE 17. The fiber that was redoped with MSA produced the mostconductive fiber, while the fiber doped with acrylic acid is the mostresistive. The data in TABLE 17 indicate that the pKa of the acidinfluences the resistivity of the redoped hollow fiber. The resistivityof the fiber remains essentially the same when the acid has a pKa ofless than 2, but the fiber resistance increases as the pKa of the acidbecomes higher. It is known that polyaniline becomes fully doped aftersufficient contact time when the pH of the acid solution is below about3. A 1.0 M aqueous solution of acrylic acid (pKa=4.25) does not havesufficient acid strength to fully dope the EB hollow fiber andconsequently, the resistivity of the acrylic redoped fiber is thehighest.

TABLE 17 Electrical Properties of Polyaniline Hollow Fibers followingAcid Redoping. Resistance after redoping Redoping Acid (Ω/cm length offiber) MSA  7.7 ± 0.3 HCl  10.0 ± 10.1 Oxalic Acid 10.4 ± 0.2 PyruvicAcid 14.8 ± 0.2 Acrylic Acid 80 ± 2

Example 8 Thermal Stability Measurements

The as-spun polyaniline fiber prepared in EXAMPLE 2 that was processedwith a stretch ratio of 1.2 (fiber #1 in TABLE 3) was first dedopedaccording to the teachings of Example 6. In a typical procedure, 6 m ofthe as-spun AMPSA doped fiber was first dedoped to its EB oxidationstate by immersing the fiber in a 0.1 M aqueous solution of ammoniumhydroxide for 30 min. After the fiber was dried for 24 h under ambientconditions, the fiber was divided into 3 approximately equal lengthsamples. In accordance with the teachings of EXAMPLE 7, the first sampleof the EB fiber was redoped with phosphoric acid by immersing the fiberin a 1.0 M solution of phosphoric acid for 24 h, while the second sampleof fiber was redoped with triflic acid by immersing the fiber in a 1.0 Msolution of triflic acid for 24 h. The third sample was not redoped. Theredoped fibers were dried under ambient conditions for 48 h. The thermalstability of the phosphoric acid and triflic acid redoped polyanilinefibers, the EB fiber, and the AMPSA-doped polyaniline fiber wasinvestigated.

The thermal stability of these fibers was evaluated by thermogravimetricanalysis (TGA) to determine their stability against weight loss (dopantloss) between 25° C. and 500° C. For these measurements, the temperaturewas varied at about 10° C./min. While the initial weight loss between25° C. and about 150° C. corresponds to loss of absorbed water from thefiber, the weight loss that is observed between 150 and 300° C. isrelated to the loss of the dopant molecules. Temperatures exceeding 300°C. resulted in the degradation of the polyaniline backbone, as indicatedby the weight loss observed for the EB fiber.

The as-spun PANI.AMPSA fiber possessed the lowest thermal stability,which can be attributed to the thermal decomposition of the AMPSA dopantmolecules at 195° C. The fiber redoped with triflic acid showed a 10° C.improvement in thermal stability, while the onset of thermaldecomposition for the fiber redoped with phosphoric acid was about 60°C. higher than that observed for the as-spun AMPSA-doped fiber.

Fibers redoped with phosphoric acid were also found to have flameretardant properties. When the fibers were placed in direct contact witha flame, they did not ignite as would, for example, AMPSA-doped fibers.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching. The embodiments were chosen and described in order tobest explain the principles of the invention and its practicalapplication to thereby enable others skilled in the art to best utilizethe invention in various embodiments and with various modifications asare suited to the particular use contemplated. It is intended that thescope of the invention be defined by the claims appended hereto.

What is claimed is:
 1. A method for exchanging dopant molecules inelectrically conductive fiber spun from a solution comprisingpolyaniline, 2-acrylamido-2-methyl-1-propanesulfonic acid anddichloroacetic acid with a selected dopant molecule, which comprises thesteps of extruding the spin solution into a coagulant, thereby causingthe spin solution to coagulate and form a fiber, and immersing theresulting polyaniline fiber in a solution containing the dopant moleculefor a time effective to achieve dopant exchange.
 2. The method forexchanging dopant molecules in electrically conductive fiber asdescribed in claim 1, wherein the coagulant is selected from the groupconsisting of esters, ketones and alcohols, and mixtures thereof.
 3. Themethod for exchanging dopant molecules in electrically conductive fiberas described in claim 2, wherein the esters comprise ethyl acetate andbutyl acetate.
 4. The method for exchanging dopant molecules inelectrically conductive fiber as described in claim 2, wherein theketones comprise acetone, methylisobutyl ketone, and 2-butanone.
 5. Themethod for exchanging dopant molecules in electrically conductive fiberas described in claim 2, wherein the alcohols comprise methanol, ethanoland isopropyl alcohol.
 6. The method for exchanging dopant molecules inelectrically conductive fiber as described in claim 1, wherein thesolution comprises an aqueous solution of phosphoric acid.
 7. The methodfor exchanging dopant molecules in electrically conductive fiber asdescribed in claim 1, further comprising the step of removing the DCAAfrom the polyaniline fiber.